Ferrofluid compositions and process of making same

ABSTRACT

A process for preparing irreversibly flocked magnetic particles, which process increases the versatility of the size-reduction process for the preparation of ferrofluids. The process comprises producing a ferrofluid in an aqueous carrier liquid with a dispersing agent by grinding coarse magnetic materials, removing the dispersing agent and attaching a different dispersing agent to the ground magnetic particles, and redispersing the particles in another carrier liquid. The process provides for the preparation of irreversibly flocked magnetic particles, and the preparation of alternate ferrofluids containing such particles.

' United States atent [1 1 osensweig Nov. 4, 1975 FERROFLUIDCOMPOSITIONS AND PROCESS OF MAKING SAME [75] Inventor:

[73] Assignee:

Ronald E. Rosensweig, Summit, NJ.

Ferrofluidics Corporation, Burlington, Mass.

22 Filed: Jan. 17, 1973 21 Appl. N0; 324,414

[52] US. Cl 252/6251; 252/6252 [51] Int. C13... HOlF 1/25; l-lOlF 1/00;C10M 3/00; CO9D 11/00 [58] Field of Search 252/6256, 62.5], 62.52,252/6254, 62.62

[56] References Cited UNITED STATES PATENTS 3,531,413 9/1970 Rosensweig252/6262 OTHER PUBLICATIONS R. Kaiser et al., Magnetic Properties ofStable Dispersions of Subdomain Magnetite Particles, J. Applied Physics,Vol. 41, No. 3 Mar. 1, 1970, pp. 1064-1072.

Primary ExaminerF. C. Edmundso Attorney, Agent, or Firm-Richard P.Crowley [57] ABSTRACT 32 Claims, 4 Drawing Figures COARSE MAGNETICSOLIDS DISPERSANT GRIND WATER FERROFLUID SOLVENT FLOCCULATING FLOCCU ESUPERNATANT AGENT WET DISPERSANT STRIPPED PARTICLES WATER-. WA

WATER AND I SOLVENT HEAT MOISTURE DRY DISPERSANT STRI PPED PARTICLES RALTERNATE $514211? GRND CARRIER LIQUID ALTERNATE FERROFLUID FERROFLUIDCOMPOSITIONS AND PROCESS OF MAKING SAME BACKGROUND OF THE INVENTIONFerrofluid is a ferromagnetic fluid displaying superparamagnetism,having a magnetic polarizability that is substantially uniform andhaving the property such that when a gradientmagnetic field is appliedto it, a body force is developed within it which can exceed by orders ofmagnitude the ordinary force of gravity on a unit volume of thematerial. Typically, ferrofluid comprises a colloidal dispersion offinely divided magnetic particles of subdomain size whose liquidcondition is remarkably unaffected by the presence of an appliedmagnetic field, and which particles resist settling under the influenceof gravitational, centrifugal, magnetic or other force fields.Ferrofluid particles typically ranging in size up to about 300 A remainuniformly dispersed throughout the liquid carrier due to thermalagitation.

Ferrofluids are described in my publication Magnetic Fluids,International Science and Technology, July 1966, pp 48-56; U.S. Pat. No.3,215,572; in the publication of R. Kaiser and G. Miskolczy, MagneticProperties of Stable Dispersions of Subdomain Magnetite Particles, J.Applied Physics, Vol. 41, No. 3, Mar. 1, 1970, pp 1064-1072; in ACatalog of Magnetic Fluids, Ferrofluidics Corporation, Burlington,Massachusetts 1972 and elsewhere.

In the prior art, a magnetic powder is reduced in size to the colloidalrange as by ball-mill grinding in the presence of a liquid carrier and agrinding aid which serves also as a dispersing agent. The dispersingagent is typically a surfactant comprising a polar long-chain moleculewhose polar group adsorbs onto the surface of the particle to produce amonomolecular protective coating that prevents particles from attachingto each other.

US. Pat. No. 3,531,413, hereby incorporated by reference, describes amethod for transferring magneticcoated particles of a ferrofluid fromone carrier liquid to another by a process of flocculation using aforeign solvent, separation of flocculated particles from supernatantliquid, and transfer of the particles into an alternate solvent in whichthe coated particles are dispersible. The carrier liquids that areemployed must be similar in their physical chemical properties. Theflocculation process is here termed reversible with the coating of adispersing agent on the particles as in the prior art remaining attachedand unchanged. As a result, the coated, flocked particles areredispersable into the 7 pure original solvent.

SUMMARY OF THE INVENTION My invention relates to an improved process forthe preparation of ferrofluids and to the ferrofluids so produced, andin particular, concerns a process for the flocculation of magneticparticles from a ferrofluid in an irreversible manner, and the treatmentof such irreversibly flocked particles to prepare improved ferrofluidcompositions.

I have discovered that ferrofluids may be flocculated by the addition offoreign agents or solvents in an irreversible manner such that thedispersant employed; that is, the surfactant that originally stabilizedthe magnetic particles in the ground ferrofluid, is removed andseparated from the ground particles. I have further found that theirreversibly flocked magnetic particles from which the first dispersantagent has been removed may be treated with another dispersant agent sothat the dispersant agent is attached to said particles, and that suchparticles may be employed to prepare alternate ferrofluid compositions.Such alternate ferrofluid compositions prepared by my process and theparticles of my process exhibit improved properties over prior artferrofluid compositions. In particular, the ferrofluid compositions ofmy invention are well suited for application in magnetic fluid seals,bearings and related devices. My alternate ferrofluid compositionsexhibit better; that is, lower, viscosity properties, while the highpacking of the volume fractions of the irreversibly flocked particles inthe ferrofluid composition permits saturation magnetization of over 300gauss. My ferrofluid composition provides for the advantages of a widetemperature range of operation, high magnetic intensity, low viscosity,oxidation resistance and colloidal stability in the presence of intensemagnetic field gradients.

My process comprises preparing a ferrofluid composition by preparing adispersion of colloidal-size magnetic particles, and a dispersant in aliquid carrier, such as by grinding through a ball-milling operationcoarse magnetic solids in the presence of a long-chain polar molecule asa surfactant and dispersant, with a water carrier. My process thenincludes removing the dispersant from the ground magnetic particles ofthe ferrofluid composition, such as by adding a foreign agent or solventto flocculate the particles, and the wet dispersant-stripped particlesremoved from the supernatant liquid. The wet dispersant-strippedparticles are then washed free of any solvent and any remainingdispersant agents,'and heated to drive off moisture and to ob tain drieddispersant-stripped irreversibly flocked particles. Such irreversiblyflocked particles are then treated with a dispersant, and redispersed inanother and different carrier liquid to provide an alternate ferrofluidcomposition having improved properties. The treatment of theirreversibly flocked, dried magnetic particles with a dispersantattaches the dispersant to the magnetic particles. The second dispersantmay be attached to the magnetic particles by grinding the particles in aball mill, together with the dispersant and the alternate carrier liquidto obtain the alternate ferrofluid composition.

My process permits the preparation of ferrofluids wherein the ratio ofmagnetic solid-core particle volume relative to the total volume of thedispersantcoated particle is greater than found in ferrofluids preparedby direct grinding processes. This greater volume permits much betterpacking of the magnetic solid particles in the liquid carrier. Forexample, a comparison of a 120 Angstrom versus a Angstrom sphericalparticle, with both having a 20 Angstrom dispersant coating, providesfor a 16 percent increase in solid packing density; however, I havefound that such increase in solid packing by my process provides forunexpected increases in saturation mgnetization over that expected forthe improved packing density.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 a, b and c are schematicpresentations of the stability of magnetic particles in ferrofluidcompositions.

FIG. 2 is a schematic process-flow sheet illustrating the process ofpreparing the improved alternate ferrofluid compositions of myinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 a, b and cschematically illustrate various states of stability of colloidalmagnetic particles of a ferrofluid composition, with FIGS. 1 a and bdirected to stability states of the particles of the prior art, whileFIG. 1 c is directed to the stability state of the magnetic particles ofmy invention, which particles have been irreversibly flocculated. Asillustrated, the magnetic particles 12 are suspended in a liquid carrier10, the carrier containing the dispersing agent for dispersant,typically, a long-chain surfactant molecule having a polar group 14,with the polar group illustrated by a circular configuration, with thelong-chain portion of the molecule schematically illustrated by a jaggedline attached to the polar circular head.

FIG. 1 illustrates the stabilized magnetic particles of the prior art,the particles showing a stabilized condition, with the surfactantmolecules adsorbed on the particle surface. The long-chain-like tails ofthe surfactant molecules are extended and well solvated with thesurrounding liquid carrier 10. The adsorbed monomolecular surfactantlayers on the adjacent particles furnish a steric hindrance, preventingthe particles surfaces from approaching close enough to agglomerate theparticles under the influence of van der Waals or magnetic forces ofattraction. The stabilized condition of the magnetic particles asillustrated in FIG. 1 a are those prepared in a typical process forpreparing ferrofluid compositions.

FIG. 1 b illustrates a reversibly flocculated group of particles whereinthe surfactant, while still adsorbed on the surface of the particles, isrecoiled to a smaller effective radius by such reversible flocculation.The shielding is incomplete with the particles loosely bound togetherunder the forces of mutual attraction. Ferrofluid compositionscontaining reversibly flocculated particles as illustrated in FIG. 1 bare typically prepared in a manner set forth in my US. Pat. No.3,531,413.

FIG. 1 0 schematically illustrates the irreversibly flocculatedparticles condition in a ferrofluid composition of the present inventionand process, wherein the surfactant molecules 14 are no longer securedto the particles surface, but instead, by my process, have been removedand are held in solution by a surrounding liquid carrier. Thesurfactant-stripped magnetic particles illustrated in FIG. 1 c andprepared by my process are more or less firmly attached to each other asillustrated, and will not redisperse in the flocculant-free carrierliquid, whereas the particles of the prior art in a reversibleflocculation as depicted in FIG. 1 b would redisperse. An importantfeature of my invention is my discovery and recognition of two differenttypes of flocculations in ferrofluid compositions containing magneticparticles, and the incorporation of such recognition into a processwhich provides particular advantages for the preparation of ferrofluidsand for the ferrofluid composition so prepared.

The materials which may be employed in the practice of my process andthe preparation of my improved ferrofluid compositions are thosematerials, such as magnetic particles, dispersants and the carrierswhich are presently employed for ferrofluid compositions. For example,the finely divided magnetizable particles include the materials usuallyrecognized as being magnetic, such as magnetite, gamma iron oxide,chromium dioxide, ferrites, such as manganese-zinc ferrite, manganeseferrite, nickel ferrite and many similar materials. Such materialsinclude also elements and metallic alloys, such as cobalt, iron, nickel,gadolinium, and Samarium-cobalt. The preferred materials for practice ofthe present invention are magnetite and gamma iron oxide. Typically,such magnetic particles are present in a ferrofluid composition inparticle size ranging from about 20 A to 300 A, with the average inparticle size being from about A to A. The magnetic particles areusually present up to about 20% by volume of the ferrofluid composition,and more typically, from about 2 to 15% by volume.

The liquid carrier employed in ferrofluid compositions in the practiceof my invention initially should be a liquid which is relativelyinexpensive, easily evaporated, of low viscosity and noncombustible,with the preferred liquid carrier for the initial stages or thepreparation of the initial ferrofluid composition being water. As morefully described hereinafter, the second carrier liquid employed in thepreparation of the alternate ferrofluid composition by my process istypically characterized by being of a relatively low viscosity, having awide temperature range for stability which would include a low pourpoint and a high flash point, and have a low volatility such that theresulting ferrofluid composition may be usefully employed and sealed ina vacuum condition on rotary bearings or similar applications. A widevariety of liquid materials may be employed as the liquid carrier ineither the first or second stage of my invention, but more typically,the second stage of the invention to prepare the alternate ferrofluidcompositions, which materials would include hydrocarbons, both aromaticand aliphatic; for example, toluene, xylene, cyclohexane, heptane,kerosene, mineral oils and the like; halocarbons, such as fluorocarbonswhich would include the fluorinated and chlorinated ethers, esters andthe derivatives of C C materials, such as perfluorinated polyethers;esters to include polyesters, di and triesters, such as azealates,phthalates, sebaccates, such as, for example, dioctyl phthalates, di-2ethylhexyl azealates, silicate esters and the like.

A dispersant or dispersing agent which is typically a surfactant whichmay be employed in my ferrofluid process and composition includes a widevariety of materials which would aid in the dispersion of the magneticparticles. Such dispersants are characterized as surfactants orsurface-active agents, and would include, for example, succinates,sulfonates, phosphated alcohols, amine long-chain acid reactionproducts, phosphate esters, polyether alcohols to includealkylphenoxypolyethoxyethanols, polyether acids and similar materialswhich are characterized by suppressing the surface tension of water, andwhich include a polar group and a long-chain tail; for example, C -C Thesurfactant is typically present in my ferrofluid compositions in a ratioof surfactant to magnetic particles of about 1:2 to 10:1 by volume; forexample, 5:2 by volume, of the solid magnetic particles.

FIG. 2 illustrates a process-flow block diagram in my process forpreparing irreversibly flocked particles and alternate ferrofluidcompositions. In my process, coarse magnetic solids, typically having asize of l to 2 microns (10,000 to 20,000 Angstroms in size), werereduced by grinding a ball mill in the presence of a surfactant-watersolution to produce a stable, colloidal, magnetizable ferrofluid, aspresently known in the prior art and as illustrated in FIG. 1 a. Thenature and behavior of ferrofluids are quantitatively described in thein sertion Ferrohydrodynamics in the Encyclopedia Dictionary of Physics,Supplement 4, Pergamon Press. The surfactant employed is chosen toproduce a ferrofluid composition which may be irreversibly flocculated.

The ferrofluid composition so prepared is then irreversibly fiocculatedby the addition of a solvent-flocculating agent, typically a polarsolvent, such as a ketone, ester or alcohol; for example, acetone ormethylethyl ketone. On the addition of the solvent-flocculated agent,the colloidal magnetic particles of the ferrofluid composition are thenprecipitated free of the liquidwater carrier, and also free of theinitial surfactant employed. The initial liquid carrier; that is, water,and the surfactant are removed in the supernatant liquid afterprecipitation of the particles, and the wet dispersantstripped particlesso precipitated recovered.

The wet dispersant-stripped particles are then waterwashed several timesto remove any traces of residual solvent or surfactant. The washedmagnetic particles are then carefully heated to drive off the moistureso as to provide dry, dispersant-stripped magnetic particles. The dry,dispersant-stripped particles are then treated with another andtypically a different surfactant, such as by coating, or as shown,grinding in a ball mill, in the presence of an alternate carrier liquidto form an alternate ferrofluid composition of my invention, and havingenhanced characteristics and properties. The alternate ferrofluidcomposition so prepared may be of the reversible or irreversible type.As illustrated, the alternate ferrofluid composition is prepared byagain grinding in a ball mill the dry, dispersant-stripped particles inthe presence of the alternate dispersant, and the alternate carrierliquid. By my process, alternate ferrofluid compositions having improvedmagnetic and viscous properties have been produced, lengthy processingtime eliminated, and more versatile ferrofluid composi- 6 tions producedwhich have alternate and improved chemical compositions.

In the preferred embodiment of my invention, the initial carrier liquidis water, while the alternate carrier liquid to prepare the alternateferrofluid composition may be water, but is preferably an organicmaterial. Further, while my tests indicate that the original surfactantemployed for the preparation of the first ferrofluid compositions may beemployed as the' alternate dispersant, the use of such surfactant hasnot been found as effective as a new and different surfactant as thealternate dispersant in preparing the alternate ferrofluid compositions.Where the same or similar carrier liquid is employed, as employed in thefirst ferrofluid composition, the alternate ferrofluid compositionsprepared by my process exhibit improved properties over the firstferrofluid compositions using such liquid carrier.

In the process of my invention, the term irreversibly flocked refers tothat condition in which the magnetic particles will not disperse in anyliquid carrier in that condition. To obtain the benefits of my process,the flocculating agent originally employed must be removed, and a newsurfactant. employed. I have found, for example, that prior artferrofluid compositions of magnetic particles and oil typically have anaverage size of Angstroms, while my alternate ferrofluid compositionsprepared by my process using the same materials have an average size ofAngstroms, and because of their condition of larger particle size,better packing is accomplished, and, therefore, better magneticsusceptibility and lower viscosity is obtained. Thus, in my process, inits preferred embodiment, the original liquid carrier employed is water,while the alternate dispersant employed should be different from thedispersant originally employed. The advantages and techniques of myprocess and] alternate ferrofluid compositions will be illustrated bythe following examples wherein Tables I and ll provide details of thematerials employed in the examples.

TABLE I CARRIER LlQUlDS Stabilizing Agent Aerosol 22 Alkanol BG AlkanolI895 Pyronate 5O Victawet 58B Polyisobutene succinic acid derivative(PIBSA) Di-Z-ethyl hexyl phosphate STABlLlZlNG AGENTS USED TO REDISPERSECOLLOIDAL MAGNETITE PARTICLES Source Description Tetrasodium N-( 1,2dicarboxyethyl )N-octadecylsulfosuccinamate American Cyanamid Co.

DuPont Co. Sodium alkylnapthalene sulfonate DuPont Co. Sodiumhydrocarbon sulfonate Petroleum sulfonate Phosphorated higher alcohol P'yUs' s 3 10):

M.W. lOOO (approx.) Reaction product with polyethylene amine WitcoChemical Corp.

Stauffer Chemical Co.

Enjay Chemical Co.

Pfaltz and Bauer lnC.

TABLE II-continued Stabilizing Agent Description Acto 500 Humble Oil &Refining Antar LM4OO GAF Corp. Chem. Div.

Alkylaryl sodium sulfonate (M.W. 460 Avg) Free acid of a complex organicphosphate ester Bryton HY Bryton Chemical Co. Synthetic sodium petroleumsulfonate Bryton Calcium 45 Bryton Chemical Co. Synthetic calciumpetroleum sulfonate Teric N8 I.C.I.A.N.Z. Ltd. Nonylphenol 8 molesethylene oxide Surfactant 157 DuPont Co. Perfluoropolyether carboxylicacid EXAMPLE 1 Saturation magnetixation Carrier liquid Magnetic particleDispersing Agent 200 gauss Distilled water (83% vol.) Magnetite (Fe O(4% vol.) Aerosol C6l (13% vol.) American Cyanamid Co.

An ethanolated alkylguani dine amine complex To the ferrofluid soprepared was added an equal volume of acetone as a flocculating agent,and the solution stirred. The colloidal particles immediatelyflocculated and began to settle under gravity. A source of magneticfield or other means may be used to accelerate the settling process. Thesupernatant liquid was decanted and discarded.

Water was added to the precipitated particles with stirring, and it wasfound that the particles did not stably redisperse; i.e., theflocculation was irreversible. This water was discarded as a supernatantand served to remove excess acetone and any dispersant that may bepresent. The wash was repeated three times, then the solids were dried.Drying was carried out under infrared lamps or in an oven at 200F untila dry black granular solid powder was obtained. Care should be taken notto heat excessively, since overheating leads to formulation of a drybrownish solid that is not suited for further processing.

The powder comprised magnetic particles of the desired colloidal sizewhich were free from any dispersant. These particles were loosely bondedtogether, but were well suited to be redispersed in an alternate carrierliquid by my process.

The dried dispersant-free powder was then combined with a surfactant andcarrier liquid, and redispersed in a ball mill in an alternateferrofluid composition as follows:

Alternate Carrier Liquid Toluene (90% vol.) Magnetic solids Fe o (4%vol.) Alternate Stabilizing Agent PIBSA (see Table II) (6% vol.)

Ball milling was carried out for a period of one to three weeks. I foundthat virtually all the magnetic solid became colloidally dispersed inthe new liquid carrier.

Of course, it is possible to ball mill directly the magnetite withtoluene as the liquid carrier and PIBSA as the dispersant to produce acolloidal ferrofluid. However, the produce is greatly inferior in itsability to be concentrated due to its higher viscosity at a given levelof magnetic saturation.

The data of Table III compares a ferrofluid of the prior art with thealternate ferrofluid of my invention. The direct grind ferrofluid wasprepared with a dibasic acid ester (see Table I), while the alternateferrofluid employed water as the direct grind carrier with the samedibasic acid ester employed as the carrier after irreversibleflocculation as described.

The comparative viscosities and ability to concentrate are typified inTable III below.

TABLE III COMPARISON OF VISCOSITY vs MAGNETIZATION MAGNETIZATION (gauss)VISCOSITY (1 cp at 30C) Ferrofluid employing PIBSA as produced in theprior art by direct grind techniques cannot be concentrated beyond about300 gauss, while my invention permits concentration of over 300 gaussand beyond 500 gauss. My improved ferrofluids may have viscosities of to2000 cps at gauss levels of 300 to 800. In fact, ferrofluid compositionswith saturation magnetization up to and in excess of 800 gauss have beenprepared by my process.

Although not wishing to be bound by any particular theory, it isbelieved that the improved properties of lower viscosity at a givenlevel of gauss for my alternate ferrofluids are based on the betterconcentrating ability; i.e., ability of particles to pack as in FIG. 10, due to the larger particle size present in preparations carried outaccording to my process. Electron micrograph study shows that theaverage particle size in Water/C61 dispersant ferrofluid is largerAngstroms) than for particles prepared by direct grinding in the systemToluene/PIBSA dispersant lOO Angstroms). Thus, at a given level ofsaturation gauss, a system of coated large particles by irreversibleflocculation takes up less bulk than a system of small particlesflocculated reversibly having the same thickness of coating, and,accordingly, the viscosity is lower.

EXAMPLE 2 Stripped dispersant-free magnetite particles obtained asdescribed in Example 1 may be coated with many other dispersing agents.A useful screening test is to add a small quantity of the strippedparticles that have been pulverized to a small quantity; e.g., 5 c.c.,of carrier liquid/surfactant solution and stir. Suitable combinations ofmaterials produce an instantaneous peptization of at least a portion ofthe magnetic particles.

My process and the above testing technique are very valuable ineliminating the time and expense associated with the filling, runningand clean-up of balls and ball mills associated with experimentalsearching for successful ferrofluid grind combinations in the past. Myscreening test requires but a few minutes and provides information thatpreviously required one to several weeks to obtain.

Table IV identifies a series of surfactant/solvent combinations thathave been identified and successfully produced in this fashion. Detailsof the surfactants and solvents are provided in Tables I and II.

TABLE IV EXAMPLE 3 For the purposes of illustration only of the rangeand nature of my invention, redispersion of stripped dispersant-freemagnetic particles from the initial aqueous ferrofluid composition ofExample 1 back into a water carrier using an alternate and differentdispersing agent has been demonstrated, employing the followingdispersants: Aerosol 22, Alkanol BG, Alkanol I89S, Pyronate 50 andVictawet 58B.

TABLE IV ALTERNATE FERROFLUIDS OF NONAQUEOUS BASE PRODUCED BY CONVERSIONOF THE WATER BASE FERROFLUID Surfactant Solvent l. Acto 500 Chevron M2V2. Antara LM400 Chevron M2V 3. Bryton HY Chevron M2V 4. Bryton Calcium45 Chevron M2V 5. Teric N8 Chevron MZV 6. Di-2 Ethyl hexyl White MineralOil Phosphate 7. Surfactant I57 DuPont E-3 The aqueous alternateferrofluid employing as a dispersant Aerosol 22 is particularly usefulin display devvices, such as those described in my US. Pat. No.3,648,269. This ferrofluid composition combines the properties ofimmiscibility with white mineral oil, colloidal stability againstfreeze/thaw, permits polystyrenesurfaces to be preferentially wetted bywhite mineral oil, and freedom from phase separation by emulsificationof the ferrofluid into an adjacent liquid phase of white mineral oil. Bycomparison, the original direct persant is effective only in the role ofsteric protection.

A further general advantage provided by the process of my invention isthe time saving in producing lots of specialty ferrofluid compositions.Thus by my process, a master batch of C6l/water base ferrofluids may beprocessed in a series of operations that require from three to six ormore months time to complete to provide a dry, stripped powder of theparticles. The powder may then be relatively rapidly converted to thedesired specialty ferrofluid in typically 1 to 3 weeks. In

fact, if complete conversion is not desired, a useful product can beoften obtained on an overnight basis in many instances. Thus, as little.as one-day processing produces a ferrofluid that previously typicallyrequired several months, or was not producible at all.

What I claim is:

1. A process for preparing an improved ferrofluid composition, whichprocess comprises in combination:

a. preparing a first ferrofluid composition comprising a dispersion ofcolloidal-size magnetic particles in a dispersant in a polar liquidcarrier;

b. adding a flocculating agent to the first ferrofluid composition toprecipitate the magnetic particles from the first composition separatingthe precipitate from the supernatant liquid to obtain flockedprecipitated magnetic particles which are characterized in that suchflocked particles will not redisperse in the polar liquid carrierwithout the addition of a dispersant;

c. recovering essentially dispersant-free magnetic precipitatedparticles;

d. coating the surface of the flocculated precipitated magneticparticles with a second dispersant; and

e. redispersing said coated particles in a second carrier liquid toprovide a second ferrofluid composition of improved properties andcharacteristics.

2. The process of claim 1 which includes drying the recovered magneticprecipitated particles.

3. The process of claim 1 wherein the second dispersant employed toobtain the second ferrofluid composition is different from thedispersant in the first ferrofluid composition.

4. The process of claim 1 wherein the second liquid carrier in thesecond ferrofluid composition is different from the liquid carrier inthe first ferrofluid composition.

5. The process of claim 1 wherein both the second dispersant and thesecond carrier liquid are different from the first dispersant and thefirst carrier liquid of the first ferrofluid composition.

6. The process of claim 1 wherein the first and second carrier liquidscomprise water.

7. The process of claim 1 wherein the first carrier liquid of the firstferrofluid composition comprises water, and the second carrier liquid ofthe second ferrofluid composition comprises a nonaqueous liquid.

8. The process of claim 1 wherein the step of coating the flockedparticles and redispersing such particles in the second carrier liquidis accomplished by grinding the recovered flocked particles in a ballmill in the presence of the second dispersant and the second carrierliquid.

9. The process of claim 1 which includes the step of washing thedispersant-free particles to remove residual traces of the dispersantfrom the surface of the particles, and, thereafter, drying the magneticparticles.

'10. The process of claim 1 wherein the first carrier liquid is water,and wherein the flocculating agent added to the first ferrofluidcomposition comprises a polar solvent.

11. The process of claim 1 wherein the second carrier liquid comprises aliquid fluorocarbon.

12. The process of claim 11 wherein the fluorocarbon comprises aperfluorinated polyether liquid.

13. The process of claim 1 wherein the second carrier liquid comprises adiester liquid.

14. The process of claim 1 wherein the magnetic particles of the firstferrofluid composition have an average particle size of from about 20 to300 Angstroms.

15. The process of claim 1 wherein the magnetic particles comprisemagnetite or gamma iron oxide.

16. The process of claim 1 wherein the second disperssant comprises, ina ratio of dispersant-to-magnetic particles, from about 1:2 to :1 byvolume.

17. The process of claim 1 wherein the second ferrofluid compositioncomprises from about 2 to by volume of magnetic particles.

18. The process of claim 1 wherein the first liquid carrier is water,and the second flocculating agent is a ketone, ester or alcohol.

19. The process of claim 1 wherein the magnetic particles of the secondferrofluid composition have an average particle size of about 120Angstroms.

20. The process of claim 1 which includes:

a. drying the essentially dispersant-free magnetic precipitatedparticles; and

b. ball milling the dry magnetic particles with the second carrierliquid and second dispersant to provide the second ferrofluidcomposition.

21. The process of claim 1 wherein the first carrier liquid is water,and the second carrier liquid is a mineral oil.

22. A ferrofluid composition which comprises a solution of alow-volatility liquid diester carrier and a dispersant, andcolloidal-size magnetic particles dispersed in said solution, thecomposition having a saturation magnetization of between about 300 to800 gauss, and having a viscosity of from about 100 to 2000 cps, themagnetic particles comprising magnetite or gamma iron oxide, thedispersant comprising, in a ratio of dispersant-to-magnetic particles,from about 1:2 to 10:1 by volume, and the composition comprising fromabout 2 to 15% by volume of magnetic particles, the magnetic particleshaving a particle size of about 120 Angstroms.

23. The flocked dispersant-free magnetic particles having a particlesize of from about 20 to 300 Angstroms produced by the process of claim1, the particles essentially free of adsorbed surfactant and incapableof being redispersed in the first carrier liquid without the addition ofa dispersant.

24. A process of preparing an improved ferrofluid composition of lowviscosity and high magnetization saturation, which process comprises incombination:

a. grinding in a ball mill coarse magnetic solids, a water-solubledispersant, and a water carrierliquid to provide a first ferrofluidcomposition of colloidal-size stable magnetic particles;

b. adding a polar solvent as a flocculating agent to the firstferrofluid composition to precipitate the magnetic particles from thefirst ferrofluid composition separating the precipitate from thesupernatant liquid to obtain flocked, dispersant-free magnetic particleswhich are characterized in that such flocked particles will notredisperse in the polar liquid carrier without the addition of adispersant;

c. washing the dispersant-free precipitated particles with water toremove the flocculating polar solvent and any residual traces of thedispersant;

. heating the washed precipitated particles to a temperature not inexcess of about 200F to remove moisture, and to produce dry, flocculateddispersant-free, magnetic particles; and

e. grinding in a ball mill the dry flocked particles in the presence ofan alternate and different dispersing agent and a second carrier liquidto provide a second ferrofluid composition.

25. A ferrofluid composition which comprises a solution of a diesterliquid carrier, a dispersant, and colloidal-size magnetic particlesdispersed in said solution, the composition having a saturationmagnetization of over 300 gauss, and having a viscosity of from about to2000 cps.

26. The composition of claim 25 wherein the liquid carrier comprises anonaqueous carrier, and the average particle size is about Angstroms.

27. The composition of claim 25 wherein the dispersant is apolyisobutene succinic acid derivative.

28. The composition of claim 25 wherein the composition comprises aratio of dispersant-to-magnetic particles of about 1:2 to 10:1 byvolume.

29. The composition of claim 25 wherein the magnetic particles comprisemagnetite or gamma iron oxide.

30. The composition of claim 25 wherein the magnetic particles of theferrofluid composition have an average particle size of from about 20 to300 Angstroms.

31. The composition of claim 25 wherein the ferrofluid compositioncomprises from about 2 to 15% by volume of magnetic particles.

32. The composition of claim 25 wherein the composition has a magneticsaturation of between about 300 to 800 gauss.

1. A PROCESS FOR PREPARING AN IMPROVED FERROFLUID COMPOSITION WHICHPROCESS COMPRISES IN COMBINATION: A. PREPARING FIRST FERROFLUIDCOMPOSITION COMPRISING A DISPERSION OF COLLOIDAL-SIZE MAGNETIC PARTICLESIN A DISPERSANT IN A POLAR LIQUID CARRIER, B. ADDING A FLOCCULATINGAGENT TO THE FIRST FERROFLUID COMPOSI-, TION TO PRECIPITATE THE MAGNETICPARTICLES FROM THE FIRST COMPOSITION SEPARATING THE PRECIPATE FROM THESUPERNATANT LIQUID TO OBTAIN FLOCKED PRECIPITATED MAGNETIC PARTICLESWHICH ARE CHARACTERIZED IN THAT SUCH FLOCKED PARTICLES WILL NOTREDISPERSE IN THE POLAR LIQUID CARRIER WITHOUT THE ADDITION OF ADISPERSANT, C. RECOVERING ESSENTIALLY DISPERSANT-FREE MAGNETICPRECIPITATED PARTICLES, D. COATING THE SURFACE OF THE FLOCCULATEDPRECIPITATED MAGNETIC PARTICLES WITH SECOND DISPERSANT, AND E.REDISPERSING SAID COATED PARRTICLES IN A SECOND CARRIER LIQUID TOPROVIDE A SECOND FERROFLUID COMPOSITION OF IMPROVED PROPERTIES ANDCHARACTERISTICS.
 2. The process of claim 1 which includes drying therecovered magnetic precipitated particles.
 3. The process of claim 1wherein the second dispersant employed to obtain the second ferrofluidcomposition is different from the dispersant in the first ferrofluidcomposition.
 4. The process of claim 1 wherein the second liquid carrierin the second ferrofluid composition is different from the liquidcarrier in the first ferrofluid composition.
 5. The process of claim 1wherein both the second dispersant and the second carrier liquid aredifferent from the first dispersant and the first carrier liquid of thefirst ferrofluid composition.
 6. The process of claim 1 wherein thefirst and second carrier liquids comprise water.
 7. The process of claim1 wherein the first carrier liquid of the first ferrofluid compositioncomprises water, and the second carrier liquid of the second ferrofluidcomposition comprises a nonaqueous liquid.
 8. The process of claim 1wherein the step of coating the flocked particles and redispersing suchparticles in the second carrier liquid is accomplished by grinding therecovered flocked particles in a ball mill in the presence of the seconddispersant and the second carrier liquid.
 9. The process of claim 1which includes the step of washing the dispersant-free particles toremove residual traces of the dispersant from the surface of theparticles, and, thereafter, drying the magnetic particles.
 10. Theprocess of claim 1 wherein the first carrier liquid is water, andwherein the flocculating agent added to the first ferrofluid compositioncomprises a polar solvent.
 11. The process of claim 1 wherein the secondcarrier liquid comprises a liquid fluorocarbon.
 12. The process of claim11 wherein the fluorocarbon comprises a perfluorinated polyether liquid.13. The process of claim 1 wherein the second carrier liquid comprises adiester liquid.
 14. The process of claim 1 wherein the magneticparticles of the first ferrofluid composition have an average particlesize of from about 20 to 300 Angstroms.
 15. The process of claim 1wherein the magnetic particles comprise magnetite or gamma iron oxide.16. The process of claim 1 wherein the second disperssant comprises, ina ratio of dispersant-to-magnetic particles, from about 1:2 to 10:1 byvolume.
 17. The process of claim 1 wherein the second ferrofluidcomposition comprises from about 2 to 15% by volume of magneticparticles.
 18. The process of claim 1 wherein the first liquid carrieris water, and the second flocculating agent is a ketone, ester oralcohol.
 19. The process of claim 1 wherein the magnetic particles ofthe second ferrofluid composition have an average particle size of about120 Angstroms.
 20. The process of claim 1 which includes: a. drying theessentially dispersant-free magnetic precipitated particles; and b. ballmilling the dry magnetic particles with the second carrier liquid andsecond dispersant to provide the second ferrofluid composition.
 21. Theprocess of claim 1 wherein the first carrier liquid is water, and thesecond carrier liquid is a mineral oil.
 22. A ferrofluid compositionwhich comprises a solution of a low-volatility liquid diester carrierand a dispersant, and colloidal-size magnetic particles dispersed insaid solution, the composition having a saturation magnetization ofbetween about 300 to 800 gauss, and having a viscosity of from about 100to 2000 cps, the magnetic particles comprising magnetite or gamma ironoxide, the dispersant comprising, in a ratio of dispersant-to-magneticparticles, from about 1:2 to 10:1 by volume, and the compositioncomprising from abOut 2 to 15% by volume of magnetic particles, themagnetic particles having a particle size of about 120 Angstroms. 23.The flocked dispersant-free magnetic particles having a particle size offrom about 20 to 300 Angstroms produced by the process of claim 1, theparticles essentially free of adsorbed surfactant and incapable of beingredispersed in the first carrier liquid without the addition of adispersant.
 24. A process of preparing an improved ferrofluidcomposition of low viscosity and high magnetization saturation, whichprocess comprises in combination: a. grinding in a ball mill coarsemagnetic solids, a water-soluble dispersant, and a water carrier liquidto provide a first ferrofluid composition of colloidal-size stablemagnetic particles; b. adding a polar solvent as a flocculating agent tothe first ferrofluid composition to precipitate the magnetic particlesfrom the first ferrofluid composition separating the precipitate fromthe supernatant liquid to obtain flocked, dispersant-free magneticparticles which are characterized in that such flocked particles willnot redisperse in the polar liquid carrier without the addition of adispersant; c. washing the dispersant-free precipitated particles withwater to remove the flocculating polar solvent and any residual tracesof the dispersant; d. heating the washed precipitated particles to atemperature not in excess of about 200*F to remove moisture, and toproduce dry, flocculated dispersant-free, magnetic particles; and e.grinding in a ball mill the dry flocked particles in the presence of analternate and different dispersing agent and a second carrier liquid toprovide a second ferrofluid composition.
 25. A ferrofluid compositionwhich comprises a solution of a diester liquid carrier, a dispersant,and colloidal-size magnetic particles dispersed in said solution, thecomposition having a saturation magnetization of over 300 gauss, andhaving a viscosity of from about 100 to 2000 cps.
 26. The composition ofclaim 25 wherein the liquid carrier comprises a nonaqueous carrier, andthe average particle size is about 120 Angstroms.
 27. The composition ofclaim 25 wherein the dispersant is a polyisobutene succinic acidderivative.
 28. The composition of claim 25 wherein the compositioncomprises a ratio of dispersant-to-magnetic particles of about 1: 2 to10:1 by volume.
 29. The composition of claim 25 wherein the magneticparticles comprise magnetite or gamma iron oxide.
 30. The composition ofclaim 25 wherein the magnetic particles of the ferrofluid compositionhave an average particle size of from about 20 to 300 Angstroms.
 31. Thecomposition of claim 25 wherein the ferrofluid composition comprisesfrom about 2 to 15% by volume of magnetic particles.
 32. The compositionof claim 25 wherein the composition has a magnetic saturation of betweenabout 300 to 800 gauss.